The moisture content of freshly harvested grain is too high for storage right away. The moisture content must be lowered before storage to prevent spoilage. For corn, a moisture content of 17 to 18 percent is considered the maximum desirable moisture content for short term storage. A moisture content of 15.5 percent is considered optimum by commercial grain elevator operators. For long term storage of the grain, 14 percent is preferred to retard spoilage. The removal of too much moisture is wasteful of drying energy and usually results in shrinkage which decreases the sale value of the grain. If the moisture is removed too rapidly, damage to the grain results.
There are a number of types of grain dryers in current use of which two major types are the continuous flow type and the batch dryer type. In a continuous flow grain dryer, grain is processed substantially continuously. Generally, large volumes of air at relatively high temperatures are passed through grain in a drying chamber or column. However, heat damage to the grain is minimized by the short stay of the grain in the chamber. Continuous flow dryers are classed as concurrent flow, cross flow, or counter flow dryers depending on the relationship between the directions of grain flow and air flow through the drying chamber. Continuous flow grain drying is capable of high rates of grain processing; however, the energy efficiency of such drying in most cases is not optimum because the drying air does not become saturated with moisture from the grain because of the high flow rate of the drying air. In more modern continuous flow grain dryers, at least a portion of the nonsaturated air is recycled through the burner before exhausting into the atmosphere to increase energy efficiency. Most commercial grain drying operations employ continuous flow systems. In addition, a fair number of individual farmers use continuous flow types of dryers.
In batch grain dryers, as the name suggests, a quantity of grain is loaded into a chamber, usually a drying bin; and heated air is passed through the grain until it is dryed. In general, it is necessary to remove dried grain in the lower layer of the bin or recirculate it to the top of the bin to prevent overdrying of the lower layer. Batch type grain drying is capable of high energy efficiency because the drying air is usually saturated with moisture from the grain before leaving the bin. However, the grain processing rates achievable in batch dryers is lower than in continuous flow dryers such that, currently, batch type drying is usually restricted to smaller operations. Batch type dryers are usually counterflow or crossflow dryers.
In most types of grain drying methods of the past, constant attention of operators was required for sampling moisture content, monitoring of temperature sensors, control of grain flow rates, and the like. Attempts have been made in the recent past to automate grain drying operations. Attempts at controlling grain drying operations through classic automatic control methods have met with varying success for a number of reasons including the diversity of types of grain drying equipment, varying environmental conditions in which such equipment is operated, difficulties in accurately characterizing the operation of grain drying, and other factors.
In the more recent past, attempts have been made to apply digital computers to the task of controlling grain drying operations. The number of variable factors which was a problem in the classic control approach continues to be a problem in the approach employed by the majority of computer controlled drying systems. This appears to be particularly true for systems in which the grain moisture content is determined indirectly by temperature measurements. The placement of temperature sensors in such systems for meaningful temperature measurements and the isolation of such sensors from environmental variables has also been a problem. For example, the placement of temperature sensors in grain unloading auger tubes has caused problems because of clogging of the tubes and damage to the sensors caused by the abrasive action of the grain passing through the tube.
Generally, moisture measurements made by measuring the dielectric constant of a grain sample in a capacitive moisture meter cell are simpler than moisture measurements based on grain or air temperatures. However, one problem with such cells as configured in the past has been that a precise volume or weight of grain is required for accurate measurements. For this reason, such capacitive moisture meters have tended to require manual loading of the grain sample in combination with complex sample size compensation techniques. For these reasons, it has generally been felt that the automatic control of grain drying operations based on capacitive moisture content measurement is not workable.